Low-loss conductor for highfrequency currents



March 18, 1947. J. SLEIQIAN 2,417,785

LOW LOSS CONDUCTOR FOR HIGH FREQUENCY CURRENTS Filed May 8, 1945 WETNESSES: INVENTOR ZZZ; Jogafi j/go/a/z BY W ATTOR Y Patented Mar. 18, 1947 LOW-LOSS CONDUCTOR FOR HIGH- FREQUENCY CURRENTS Joseph Slepian, Pittsburgh, Pa, assignor to Vtestinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Application May 8, 1943, Serial No. 486,189

9 Claims.

This invention relates generally to electric conductors, and particularly to conductors designed for high-frequency currents and means for improving the electrical characteristics thereof by reducing losses due to skin-effect.

Electrical conductors carrying alternating current exhibit the property that the current concentrates around the outer portion of the conductor. This increases the current density near the surface and leaves the inner portion of the conductor to act merely as a physical support which takes little part in the current carrying capacity. The phenomenon of skin-effect is at tributed to the fact that the flux generated by the current also generates a counterelectro-motive force in the conductor which is greater in the center thereof than on the adjacent surface. The current concentration near the outer surface of the conductor increases its effective resistance to the flow of alternating current. This increase is comparatively small for currents of the usual power frequencies, termed low-frequencies, but becomes quite pronounced at higher or radio frequencies.

A particular feature of this invention is that simple means are provided for increasing the effective current carrying surface of the conductor at the particular frequency.

Another feature of the invention is that the method in accordance with this invention for increasing the effective current carrying surface can also be applied to existing conductors of various sizes and shapes.

An advantage of the invention is that by increasing the current carrying surface a substantial reduction in losses is obtained which results in material saved which otherwise would be used for the conductors.

A further advantage resulting from the invention is that efficient low loss conductors can be supported by non-conducting bodies whereby only the minimum amount of conducting material is required for efficient current transmission.

Other objects and advantages will be apparent from the following description of the invention, pointed out in particularity by the appended claims and taken in connection with the accompanying drawing, in which:

Figure 1 shows a greatly magnified view of the surface of a conductor prepared in accordance with this invention,

Fig. 2 is a similar View of a modified form,

Fig. 3 is an elevational view of a hollow conductor, the current carrying surface of which is efiectively increased,

-ig. l shows the application of the invention to a flat ribbon-like conductor, and

Fig. 5 shows the use thereof to provide an efii cient conductive surface over non-conducting bodies.

As stated before the resistance of conductors to high frequency is. high because of skin-effect. In a good conducting material such as copper, for example, it was found that the effective skin-effeet depth is equal to 1% divided by the square root of the frequency of transmission. In other words, skin-effect where f is the frequency in cycles and the depth d is in centimeters. For different frequencies it can easily be shown that the effective conducting surface rapidly diminishes as the frequency increases. Following the above formula, it is seen that at a frequency of 1 megacycle the skin effect depth is .01 centimeter whereas at 100 megacyclcs it is .001 centimeter. In accordance with this invention, it is proposed to reduce the losses due to skin effect by proper slotting of the conductor in the direction of current. Calculations have shown that to get a reduction of losses, a certain proportion must be maintained between the depth and the thickness of the slot and correspondingly between the thickness of the fins which result due to slotting.

Referring to Fig. l, the enlarged view intends to convey the desired relationship. It was found that to get a reduction of losses, the Width b of the slot should not be too large nor too small compared to the skin efiect depth. The calculations which proved advantageous in practice involved the approximation depending on the ratio of where h equals the width of the slot and d the depth of skin effect. It was found that when this ratio equals 10, the losses for copper can be substantially reduced in the ratio of .72. It was also found that a considerably larger value can be given to 0 without substantial increase in losses. With ll) as the value for c thewidth a of the fin should be approximately 10% of the zidth b of the slot, whereas the depth 6 of the slot should be 15 times the efiective skin effect depth d.

In accordance with the above proportions which give an approximate reduction of losses of 3 72%, the following values may be plotted for different frequencies:

ductor shown in Fig. 1 shows a rectangular groove. However, the current densities near the bottom of the slot are quite low so that the slot may be narrowed at its base which will make the fins more rigid as shown in Fig. 2 without substantially affecting the losses. From the table given above, it is seen that the dimensions of the slots and the fins at the higher frequencies is extremely small and at very high radio frequencies of only microscopic character. These slots can, therefore, be made very simply by a roller tool properly hardened which can be applied to the copper surface at a temperature where plastic flow is rapid.

Fig. 3 shows a hollow conductor the surface lines represent slots running axially in the direction of current flow.

The invention is particularly applicable to sheets or ribbons of conducting material, such as copper or silver, which may be grooved transversely, longitudinally or on the bias depending upon the application of the conducting ribbon. Fig. 4 shows a ribbon material grooved transversely at a proper angle so that the ribbon may be taped around metallic or insulating supports and thereby produce a low loss conductor of sufficient physical strength. Fig. 5 indicates how the ribbon of Fig. 4 may be wound around a rod G of non-conducting material in such manner that the lines forming the grooves will run coextensively forming a continuous channel coaxially with the support G. In order to make a good overlapping surface, the slots in Fig. 4 are not extending all the way to the edge of the ribbon but a plane surface H is left on one edge to cooperate with the groove H edge of the next convolution of the ribbon when it is wound over a support, as shown in Fig. 5. Ribbons of conducting material prepared in accordance with this invention make efficient conductors and can be utilized also i to line cavity oscillators or any other surface where it is desired that high frequency resistance should be lowered to the minimum. For the very high frequencies the thin fins may be made more durable by utilizing non-oxidizing materials or a coat of protecting lacquer.

I claim as my invention:

1. An electrical conductor in the form of a ribbon of flexible conducting material of a thickness greater than the skin effect depth at the frequency of the current to be conducted, slots cut in the conducting surface of said ribbon transversely to its length, said ribbon being wound on a support to form a solid conductive body, said transverse slots coextending in the direction of the current flow coaxially of said support.

2. A conductor for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin effect depth for said current.

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3. A conductor for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin effect depth for said current, the width of said surface between edges of adjacent slots being substantially equal to said depth.

4. A conductor for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin effect depth for said current, the depth of said slots being substantially 15 times said skin effect depth.

5. A conductor for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin effect depth for said current, the width of said surface between edges of adjacent slots being substantially equal to said depth, the depth of said slots being substantially 15 times said skin effect depth.

6. A conductor having about the conductivity of copper for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin effect depth for said current.

7. A conductor having about the conductivity of copper for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin effect depth for said current, the width of said surface between edges of adjacent slots being substantially equal to said depth.

8. A conductor having about the conductivity of copper for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin efiect depth for said current, the depth of said slots being substantially 15 times said skin effect depth.

9. A conductor having about the conductivity of copper for high frequency alternating current having slots on its surface parallel to its longitudinal axis, the width of said slots being substantially 10 times the skin eifect depth for said current, the width of said surface between edges of adjacent slots being substantiall equal to said depth, the depth of said slots being substantially 15 times said skin effect depth.

JOSEPH SLEPIAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 290,769 Gray Dec. 2-5, 1883 123,958 Welsh Feb. 20, 1872- 543,960 Gray Aug. 6, 1895 2,356,044 Foulkes Aug. 15, 1944 FOREIGN PATENTS Number Country Date 546,415 British July 13, 1942 584,982 German Sept. 27, 1933 474,267 German Apr. 2, 1929 456,364 German Feb. 21, 1928 2,702 Netherlands Dec. 2, 1918 

